Field of the Invention
The invention relates to a control system for controlling the rotational speed of a turbine for producing electrical power, including a first control structure for controlling the rotational speed during idling and/or insular operation of the turbine. The invention also relates to a method for controlling the rotational speed of a turbine during load shedding.
In the case of turbines, in particular turbogenerator sets for producing electrical power, it is frequently necessary for operational reasons to prevent rapid shutdown during load shedding, that is to say while the electrical power to be emitted is being reduced. Rapid shutdown of the turbine occurs, for example, if the rotational speed of the turbine after load shedding exceeds a critical value, for example 10% of the nominal rotational speed. The special control-engineering measures required for prevention can be implemented by the use of additional so-called sudden load-change devices. Those sudden load-change devices cause a control valve which controls the rotational speed to close immediately during load shedding, with the period for which the control valve is kept closed being dependent on the magnitude of the load shed, that is to say on the reduction in the electrical power produced by the turbine and the associated generator. After that period has elapsed, the control of the rotational speed is taken over by the rotational-speed controller of the turbine. That controls the rotational speed during idle operation and during insular operation of the turbine, that is to say in the case in which no electrical power is being emitted to a power supply network. During network or on-line operation, in which electrical power is emitted to a power supply network, the turbine is coupled to the generator, so that the rotational speed of the turbine is governed by the nominal rotational speed (synchronous rotational speed) and the rotational-speed controller is used to control, for example, the fuel supply in the case of a gas turbine or the steam supply in the case of a steam turbine.
German Published, Prosecuted Patent Application 1 296 657 describes an electro-hydraulic controller for the fresh-steam valve of a steam turbine. The fresh-steam valve is controlled by an opening controller, as a function of its position. A parameter which is dependent either on the rotational speed or on the power is used as the setpoint value for control purposes. Pure control of the rotational speed is carried out in the event of load shedding, that is to say when the synchronous generator is suddenly disconnected from the joint network. A sudden load-change relay, which is not described in more detail, is used for that purpose.
Published UK Patent Application GB 2 011 126 A, corresponding to U.S. Pat. No. 4,238,924, describes a control system of a gas turbine. In that control system, a fuel supply valve is controlled by a PI controller. That control system operates in pure network operation of the gas-turbine system and is constructed in such a way that it ensures appropriate follow-up in the event of load changes. Such control systems have an inherent conflict which is that the integral part requires a long time constant for good control quality and, in contrast, requires a short time constant in order to react as quickly as possible to a change in the load on the gas turbine. The selected time constant of the integrator part results in undesirable fluctuations in the rotational speed of the turbine even if an additional fuel return valve is added, through which fuel is fed back in the event of a load change, and the quantity of fuel flowing into the combustion chamber is thus reduced. Rapid matching of the fuel flow rate to the rotational speed required after a load change, that is to say stabilizing the rotational speed at the required constant rotational speed as quickly as possible, is carried out by superimposing a constant signal on the rotational-speed difference signal. That speeds up the response of the integrator to the difference signal. The rotational-speed difference signal is applied continuously to the function generator, in order to control the rotational speed. The function generator thus reacts to a change in the rotational speed, corresponding to the rotational-speed difference signal which differs from zero, in such a way that the fuel flow rate being supplied is reduced through the fuel supply valve.
German Published, Non-Prosecuted Patent Application 26 27 591, corresponding to U.S. Pat. No. 4,146,270, specifies a control device for turbines having a rotational-speed controller and a power controller. The two controllers are connected to a minimum selection structure and are slaved to one another during network operation of the turbine. The respectively smaller value of the rotational-speed controller or of the power controller is selected by the minimum selection structure and is supplied to a proportional element, which produces a valve control value. The rotational-speed controller is constructed in such a way that, if a sudden load disconnection takes place, the resultant increase in the rotational speed of the turbine is kept small such that, in particular, a rise of only about 1% of the operating rotational speed occurs. As a single specifically described embodiment, a rotational-speed controller is specified which has a proportional element, an integral element and a differential element. When there is no load on the turbine from the generator, the output of the rotational-speed controller is immediately driven downwards through the differential element, and the turbine is stabilized at a fixed speed of revolution. The speed of revolution, which rises in the event of load shedding, is thus immediately reduced again by the differential section. To that end, the mains frequency is supplied directly to the differential section as a signal, and the difference between the required frequency and the mains frequency is supplied only to the proportional section and the integral section.